Party line telephone system



y used in common by a plurality of stations.

Patented Nov. 12, 1940 Y.

PARTY LINE TELEPHONE SYSTEM Karl Schiifmann, Berlin-Wilmersdorf, and Emil Walter, Berlin-Lubars, Germany, assignors toy Telephon-Apparat Fabrik E. Zwietusch and Company, Charlottenburg, Berlin, Germany Application October 14, 1937, Serial No'. 168,870 In Germany October 26, 1936 4 Claims.

The invention relates to a circuit arrangement for telephone stations with limi-ted call facilities which are to be operated over a connecting line stations which are restricted as regards the calls which they can establish are equipped in the known manner with dials whose impulse sending cont-acts are blocked when particular call numbers are dialled. With this arrangement it is possible to avoid the restriction by displacing the dial plate before the connecting up of the switches so that after the switch has been connected the forbidden call, such for example as the selection of the distant exchange, can be effected. In the case where the station whose call facilities are limited is connected directly to the circuit in the exchange this possibility of fraud is negligible in practice because the time for the connection of the switches by the preselecting device is so short that it is not possible within this period to displace the dial plate without simultaneously setting the public exchange switches. If, however, the station is connected to a line which is to be used in common with other stations it is very easy to displace the dial plate without simultaneously setting the switches, if the calling party takes up his receiver when the line is already engaged and operates the dial. In this case the calling party can displace his dial switch without at the same time influencing the switches. As soon as the line has become free the calling party sets up the forbidden call.

rIhe present invention aims at preventing the possibility of fraud and realizes this aim in that when the public exchange line is engaged, switching means become operative when a station hav-v ing restricted facilities makes an attempt to set up a call and prevent the station from connecting up after the line has become free until all the switching means at the station are restored to normal by the replacement of the receiver.

An embodiment of the invention is represented in the drawing. The drawing sho-ws a party line system with three branch stations. The station M is a pay station from which only local calls may be set up in the incoming and outgoing directions while the stations NI and N2 are stations with unrestricted service. The station M is provided with a dial switch in the known way which is equipped with special auxiliary contacts which become operative when particular call numbers are dialled. In the embodiment it is assumed that the more highly rated calls which may not be established by the pay station possess a nought as the rst digit of their call numbers.

The

The dial is therefore provided with an auxiliary Contact O which closes as soon as a nought is dialled as first digit of va call number. Besides this auxiliary contact O the dial possesses a socalled level contact dk which is switched over 5 as soon as the dial is pulled off-normal and remains` in this position until the receiver is replaced. mi represents the dial impulse contact and ma the dial olf-normal contact. The switchliook which controls the contacts IHU and 2HU 10 is coupled with a delay device in the known way to prevent impulses ybeing transmitted by means of the switchhook. Mi represents.` the microphone and F the receiver. At each station a visi-r ble signal SZI, SZZ and SZS is disposed. The 15- paystation lVI is arranged as a party line station and therefore possesses a calling bell W. The station M is further provided with relays K Aand S of which relay S isv provided with two windings SI and SII. On calls in the incoming direction 20 the station M is ready for conversation immediately the receiver is taken up since relay K pulls up over contact EHU and connects the paystation M to the line Ld, Lb; over contact llc and 219. The busy relay B is energized over busy contact 31e 25 and this at its contact b operates the busy signals SZI, SZ2 and S23 at the individual stations. Relay S also energizes over winding SI over contact 3k. The opening ofcontact is remains without eiect on relay K since this relay holdsv up over 30 its contact 57o.

If a call is to beset up in the outgoing direction by the paystation at a time when the line is not engaged relay K operates first over the switchhook contact ZHU as before. first digit of the call number is dialled the contact dic short-circuits the microphone Mi so that after the called party has replied the calling party is constrained to insert a coin in the apparatus in order to switch over contact KT. After the coin 40 has been collected the impulse contact mi is shortcircuited at contact dic and contact KT so that the calling party is forced to replace his receiver before setting up a fresh call in order to restore the contacts dic and KT to normal.. If the calling 45 party attempts to set up a forbidden call having a 0 as the first digit of the call number the impulse contact usi is short-circuited by contact O which prevents dialing.

If one of the stations NI and N2 has already 50 set up a call relay B and relay S are energized over the contact 3H! or 3H2. The energizing circuit for relay K is broken by contact Is so that after the receiver at the station M has been taken up when the line is already engaged relay K can- 55 When the 35 v not energize. Relay S, however, looks up in this case over contacts is and 4k and its Winding SII in dependence upon the switchhook contact ZHU. If When the line is engaged the calling party at the station M should attempt to displace the dial plate by dialling a ldigit which is not blocked in order to be able to dial the blocked digit 0 as soon as the line becomes free this is without efiect be,

cause relay S remains energized and prevents the operation of relay K until the receiver is replaced. The replacement of the receiver, however, renders the displacement of the dial plate already eilected inoperative.

What is claimed is:

l. In a telephone system, a plurality of subscribers substations, a party line serving said substations and capable of being taken into use by any of the substations to extend a call, means at one of said substations operable by a subscriber thereat for initiating a call from said one substation, and means at said substation, operated if said call initiating means is operated at a time when another oi said substations is engaged in a call over said line, to prevent said one substation from taking said line into use until said call initiating means has been released and reoperated at a time when said line is free.

2. In a telephone system, a party line, a plurality of subscribers substations served by said line, means at each of said substations operable by a subscriber thereat for initiating a call from that substation, each of said substations normally effective responsive to the operation of said call initiating means thereat to take said line into use toextend said call, means at one of said substations for preventing that substation from taking said line into use if said line is engaged in another call, and means, operated by said call initiating means at said one substation and maintained operated thereby if said call initiating means is operated While said line is engaged in another call and said line thereafter becomes free before said call initiating means at said one substation is released, said last means effective to prevent said one substation from taking said line into use until after said call initiating means thereat has been so released.

3. In a telephone system, a party line serving a plurality of substations, a relay at one of the substations, means for operating said relay responsive to another of said substations becoming engaged in a call over the line and for maintaining said relay operated during the call, call initiating means at said one substation operable by a subscriber thereat, said call initiating means effective, if operated during the call in which said other substation is engaged and not restored before the termination of said call, to maintain said relay operated after termination of said call, and means controlled by said relay to prevent said one substation from gaining access to said line after the termination of said call until said relay has been restored by the restoration of said call initiating means.

4. In a telephone system, a party line serving a plurality of substations, a normally restored relay at one of the substations, means for operating said relay responsive to another of said substations becoming engaged in a call over said line and for maintaining said relay operated during the call, call initiating means at said one substation operable by a subscriber thereat, and means controlled conjointly by said call initiating means and said relay to take said line into use for a call from said one substation only ii said call initiating means is operated by the subscriber at a time when said relay is restored.

KARL SCHIFFMANN. EMIL WALTER. 

